It is estimated that from 20 to 35 million tons of sulfur dioxide were vented to the atmosphere in the United States in 1972 from industrial plants and power plants by burning fossil fuels containing bound sulfur. As a result of the ambient air quality standards and pollution control laws, there have evolved an estimated fifty different processes for removal of the sulfur from the fuel or for clean-up of the SO.sub.2 from the flue gases resulting from the fuel combustion.
Principal among these processes are wet scrubbing systems employing an alkaline additive which reacts with the SO.sub.2. The most utilized wet scrubber additive has been a calcium based material such as lime or limestone. In these processes, the calcium oxide, hydroxide or carbonate reacts with the SO.sub.2 to form calcium sulfate. The calcium sulfate is relatively insoluble and may be removed from the waste water and disposed of in sludge ponds.
However, calcium systems and scrubbers pose several problems. Initially, the energy required to pump the water through the scrubber is relatively high as compared, say, to a baghouse system which uses air rather than water. In addition, scrubbers are prone to scale formation due to the ash from fuel such as coal collected in the scrubber water. The scaling problem may be complicated by the precipitation of calcium sulfate in the scrubber or downstream demister. Further, the relatively low reactivity of a calcium system, as compared to a sodium system, means that the liquid/gas ratio must be relatively high, the concentration of the calcium alkali in the solution is relatively low, and there must be recycle of unreacted alkaline calcium compounds. The recovery of the calcium sulfate as a sludge from the scrubber water is complicated by the inverse solubility exhibited by that compound. Thus, when the hot flue gases come in contact with the scrubber water, the calcium sulfate is less soluble at higher temperatures and tends to precipitate in the scrubber rather than in external separation tanks.
In order to overcome the inverse solubility problem, the concentrations in the scrubber water must be kept low and this results in low efficiency. In addition, the external settling tanks should be kept heated, which in northern climates is a relative impossibility.
Sodium and ammonium alkaline compounds have been proposed for wet scrubbers. These compounds have the advantage of increased reactivity and a low liquid/gas ratio due to the fact that the end products sodium or ammonium sulfate/sulfite are highly soluble in the scrubber water. Although these compounds do have a degree of inverse solubility, the sodium or ammonium sulfate is so highly soluble even at the warmer scrubber temperatures that it does not precipitate in the scrubber.
However, the solubility of the end products sodium or ammonium sulfate and sulfite have prevented the adoption of such systems since the disposal of soluble products poses potential water pollution problems. To avoid trading water pollution for air pollution, there have been proposed so-called double-alkali processes in which a sodium or ammonium system is used in the scrubber in order to avoid the plugging problems, and the sodium or ammonium sulfate is reacted externally with lime to produce insoluble calcium sulfate.
However, such double-alkali processes have met with practical difficulty insofar as the excess oxygen present in the flue gases of power plants and the like have been sufficient to produce predominantly sodium sulfate. The sodium sulfate, as contrasted to sodium sulfite, does not react well with calcium oxides, hydroxides or carbonates to produce the insoluble calcium sulfate.
As a result of the inherent complexities and high energy demand of scrubbers, there have been proposed dry systems for SO.sub.2 control. The best of the dry systems involves injection of a dry additive material just upstream of a baghouse. The baghouse serves two purposes, to collect the dry additive material, thereby providing a site for reaction of the SO.sub.2 with the additive, and also to function as a particulates filter aid for fly ash, processing dust and the like in the flue gas. While many dry sorbents have been tried, the sodium-containing dry additives appear to be far more reactive than any others, including calcium-based additives.
These sodium-type dry additive materials react in the baghouse to form sodium sulfite or sulfate with other percentages of persulfites (pyrosulfites), persulfates (pyrosulfates), bisulfates and bisulphites as the case may be. The resultant cake, hereinafter called a sulfate cake for simplicity, poses a potential water pollution problem in connection with its disposal. Although these cakes can be successfully landfilled, the problem is primarily economic since to prevent leaching of soluble sodium sulfate from the fly ash-containing sulfate cake, the landfill must be specially constructed and continuously monitored to meet statuatory limitations on leaching and ground water contamination. As a result of the water pollution problem posed by the sulfate cake waste products, no sodium system dry additive baghouse injection processes have been adopted for pollution control although they appear to be the least expensive SO.sub.2 control system proposed to date.
Non-ferrous metal smelters are a principal source of SO.sub.2 pollution from off-gases of roasters, reverbatory furnaces and converters. Recent estimates indicate that copper smelters alone account for some 3.6 million tons of SO.sub.2 emissions, of which only 600,000 tons are being recovered as sulfuric acid. The two oldest flue gas desulfurizing processes that were specially developed for treating lean or less concentrated SO.sub.2 smelter gas streams are the Cominco ammonia process in use since 1916, and the Asarco dimethylanaline process developed in the 1950's.
The Cominco process uses ammonia for sequestering sulfur oxides from lean (0.6 to 2.0 percent SO.sub.2) streams and reduces exit sulfur oxides concentration to some 1000 ppm. The process recovers almost 100 percent concentrated sulfur dioxide stream and, as a byproduct, produces ammonium sulfate. In early Cominco operations, strong or 100% concentrated sulfur dioxide streams were converted into elemental sulfur by a so called "incandescent carbon" process. Later, when Cominco undertook production of fertilizers, all recovered sulfur dioxide was converted into sulfuric acid, the situation that presently exists.
Technically, the Cominco process, in combination with the conversion of strong sulfur dioxide streams into sulfuric acid, would provide required sulfur level control in most of existing copper smelters, if ammonium sulfate and sulfuric acid disposal problems could be satisfactorily resolved. The performance of this process on a copper reberberatory furnace has, however, not been commercially demonstrated.
Asarco's dimethylaniline process is another potential sulfur oxides control method that was aimed to desulfurize lean smelter flue gas streams containing greater than 1.5 percent sulfur dioxide concentration. In its original process configuration (identical to Sulfidene process in Germany), the DMA process was operated at Asarco's Selby smelter in California for a number of years while reaching its highest recovery capacity of some 40 tons SO.sub.2 per day. In 1971 the Selby smelter and with it the DMA process was shut down. Since DMA as a sulfur oxides sequestering agent capacity-wise is inferior at low sulfur oxides concentration to xylidine (sorbent used in Germany), in 1970 Asarco undertook DMA process modifications which are still in progess.
The DMA process is capable of reducing sulfur oxides concentration in the purified gas to about 500 ppm while producing almost 100 percent sulfur dioxide stream for conversion into sulfuric acid or into liquid sulfur dioxide product. Besides, the original DMA process produces another byproduct, sodium sulfate, at a rate of some 45 lbs/ton of recovered SO.sub.2. Recent DMA process modifications, as far as it is known, are related to avoiding production of sodium sulfate byproduct and attempting to dispose sulfates in the form of gypsum.
Since the DMA process has never gained acceptance by industry on a large, industrial scale, and is still undergoing further development, it can not be considered as being readily available for treating lean sulfur oxides streams at existing copper smelters. If successfully developed, the DMA process in combination with sulfuric acid plants could render adequate sulfur oxides emission control at majority of copper smelters.
Other present day lean flue gas desulfurizing processes to be mentioned are sodium sulfite/bisulfite systems, and lime/limestone based systems, the latter of which was discussed above. The first system is regenerable one, producing a concentrated sulfur dioxide stream, while the second does not recover sulfur values, i.e. is nonregenerable.
The sodium sulfite/bisulfite system is capable of desulfurizing flue gas streams containing from 0.02 to 1.5 percent sulfur dioxide. In case of oil fired boiler flue gas or tail gases from sulfuric acid or Claus plants, this system is capable of reducing sulfur oxides concentration in the exit gas to less than 50 ppm. As with all sulfite/bisulfite systems, there is an appreciable sulfate purge stream (equivalent to some 5 to 25 percent of incoming SO.sub.2) subject to disposal. In combination with sulfuric acid plant, treating strong smelter streams, this process, if specially developed for smelters, could provide an adequate sulfur oxides control for meeting AAQS (ambient air quality standards).
In all three of the above-proposed smelter processes there is a presently unsolved disposal problem, of ammonium sulfate in the Cominco process, and of sodium sulfate in the Asarco DMA process, the Sulfidene process and the sodium sulfite/bisulfite process. The sulfate insolubilization process of this invention removes a significant barrier to the commercial adoption of the above known smelter SO.sub.2 control processes.
To date we know of no success of others in the art to devise a system for the insolubilizing of sulfate cake or sulfate-containing wastes so that sodium based systems may be used for SO.sub.2 control. Indeed, we have been told that until the problem of insolubilizing the sodium or ammonium sulfite/sulfate containing wastes at an economic level is solved, sodium or ammonium systems, wet or dry, will not be adopted by utilities or industry for flue gas SO.sub.2 emission control, even though such systems are cheaper.
In the unrelated hydrometallurgy art, proposals have been made to remove metals, e.g. iron and/or aluminum, alone or concurrently, from copper dump leachate solutions.
Thus, for example, U.S. Pat. No. 2,296,423 discloses a method whereby acid solutions containing iron (or iron and aluminum) are subjected to high temperature and pressure in an autoclave to hydrolize sulfates of ferric iron and aluminum and precipitate basic salts with a simultaneous generation of free acid. According to the patent, oxidation of iron to the ferric state is promoted by the direct injection of oxygen into the solution while the solution is at high temperature and pressure. The patent further teaches the addition of an alkali salt, e.g., sodium sulfate or sodium chloride, to promote the precipitation. Soluble iron oxides are added to partly consume the free acid generated in the autoclave operation. The precipitate formed under the conditions taught by the patent is disclosed to be a double basic salt of alkali metal and iron, Na.sub.2 SO.sub.4.3Fe.sub.2 (OH).sub.4 .SO.sub.4, somewhat analogous to, but apparently different from the natural mineral Natrojarosite, NaFE.sub.3 (SO.sub.4).sub.2 (OH).sub.6. The patent further teaches that if aluminum is present in the solution both the iron and aluminum can be nearly completely precipitated as a complex basic iron aluminum alkali sulfate.
U.S. Pat. No. 3,434,947 is directed to separation of iron from zinc sulfate solutions produced in hydrometallurgical leaching of "Calcine," a roasted sulfide ore concentrate. The iron is precipitated in the presence of K.sup.+, Na.sup.+, NH.sub.4.sup.+ ions in a concentration of 1/10 to 1/4 the amount of the iron content in g/l. Ferrous ion is oxidized to ferric ion by MnO.sub.2 and the solution is partly neutralized with ZnO prior to the precipitation. The basic iron sulfate precipitated is described in related U.S. Pat. No. 3,684,490 as being jarosite and the K, Na and/or ammonium source being an unnamed salt.
Sideronatrite, Metasideronatrite, Natrojarosite, and Ammoniojarosite are found in nature (See Palache, C.; Berman, H.; Frondel, C.; Dana's System of Mineralogy, Vol. II, John Wiley & Sons, 7th Ed., 1951, pp 562, 563, 603, 604). However, the conditions under which formation of these compounds occurred in nature is unknown. Scharizer, in Za.Kr., Vol 41 (1906) p. 215 reports formation of Sideronatrite by slow precipitation at room temperature over a period of months. Mellor, A Comprehensive Treatise of Inorganic & Theoretical Chemistry, Vol 14, p. 345 (1935) Longmanns Green & Co., reports on work by Skrabal, A., Zeit. anorg. Chem., Vol. 38, (1904) p. 319, as preparing Sideronatrite under conditions of high Na, Fe and SO.sub.4 concentration by heating sodium and ferric sulfate in the presence of sulfuric acid on a hot plate. The above appears to us to be the most relevant prior art on the subject of which we have present knowledge.